背
せい · sei · JLPT N5 · noun
height
📏 sei sounds like say: say your height — same kanji as back (背中)
背 is the back of the body, built from 北 (two people back to back) over 月 (flesh). Height is measured along the back, so 背 read sei (or se) came to mean stature, as in 背が高い = tall.
背 — north 北 (phonetic) + flesh 月/肉
Originally a phono-semantic compound: 月 (a form of 肉, 'flesh/body') gives the meaning 'back of the body,' while 北 supplies the sound. Interestingly, 北 itself depicted two people back-to-back and once meant 'back'; when it was borrowed for 'north,' 背 was created with the flesh radical to keep the original sense.